I am imagining treating the last commit as your staging area. Am I missing something that makes that impossible? Specifically, what is the operation that you can't map from staging to a final commit?
Unstaging, maybe? I use SourceTree, so I don't have to remember the specifics of staging-related commands vs commit-related commands. It's just right-click > "Stage / Unstage File", click "Stage / Unstage Hunk" or shift-click "Stage/Unstage Lines".
Undoing part of a commit to be the equivalent of an "Unstage" action is feasible, I'm sure, but I couldn't tell you off the top of my head exactly what that command would be.
So overall, part of it is that it's a mental model, part of it is that the concept is built right into Git.
